Did the asteroid that hit the Earth, creating the Moon, originate from one of Earth's Lagrangian points? (ESA)
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Two solar telescopes launched to study coronal mass ejections and the solar wind have been sent to do an entirely different task. Currently, the Solar Terrestrial Relations Observatory (STEREO) probes are flying in opposite directions; one directly in front of Earth’s orbit and the other directly behind. This unique observatory is intended to view the solar-terrestrial environment in unprecedented detail, allowing us to see the Sun from two vantage points.
This might sound like an exciting mission; after all, how many space-based observatories have such a unique perspective on the Solar System from 1 AU? However, both STEREO probes are currently moving further away from the Earth (in opposite directions), approaching a gravitational no-man’s land. STEREO is about to enter the Earth-Sun Lagrangian points L4 and L5 to hunt for some sinister lumps of rock…
The Lagrangian points of a two-body system, such as the Earth and the Sun.Lagrangian points in planetary systems are islands of gravitational stability. They are volumes of space where the gravity of two massive bodies cancel out. The first two Lagrangian points in the Earth-Sun system are fairly obvious. The L1 point is located directly between the Earth and Sun, about 1.5 million km from the surface of the Earth, the point at which the gravitational pull of the Sun and Earth cancel each other out.
The L2 point is located at approximately the same distance, but on the opposite side of the Earth. In this case, the Earth is constantly eclipsing the Sun. The L3 point is on the opposite side of the Sun from the Earth, at approximately 1AU. Now this is where it starts to get a little strange. The L4 and L5 points are located 60° in front and 60° behind the Earth’s orbit. The 4th and 5th Lagrangian points are also the most gravitationally stable regions, primordial debris lurks, trapped in the Lagrangian prisons. Although the L1 point is often considered to be the most stable of the Lagrangian points (as it’s directly locked between the gravity of the Sun and Earth), even space observatories (such as SOHO and ACE) have to carry out complex orbits to remain in place. Otherwise the delicate balance will be lost and they will drop away from L1.
L4 and L5 are in fact the most stable locations, balanced by a complex cage of competing gravitational components from the Earth and the Sun. It is thought that these two regions have trapped lumps of rock and dust all the way through the evolution of the Solar System, making them a very interesting place to send a space mission. And the two solar probes of STEREO are currently racing toward L4 and L5, about to explore the gravitational dead zone, whether they like it or not.
It is a known fact that other planets in the Solar System possess these islands of gravitational calm, and asteroids have been observed sitting in stable locations in front and behind of Jupiter’s orbit for example (called “Trojans” and “Greeks”). Does Earth have a swarm of asteroids sitting in its L4 and L5 points? Scientists believe this is a certainty. However, no asteroids have ever been observed.
Although millions of kilometres across, L4 and L5 can only be observed at dawn and dusk. Any possibility of spotting a large asteroid diminishes rapidly as they are obscured by the Sun. So, the STEREO space telescopes are going to take the dive into L4 and L5 to see, first hand, what lies in wait.
Artist impression of the STEREO probes going their separate ways (NASA)Early on in the STEREO mission, scientists discussed the possibility of stopping the spacecraft inside the two islands of calm to provide an advanced warning of incoming charged particles from coronal mass ejections during solar maximum. However, slowing the craft down would have cost the mission too much fuel, so the decision was made to let the solar telescopes pass straight through. It will take a few months to complete the journey through the huge Solar System badlands, but it will serve a valuable purpose, STEREO has become NASA’s makeshift asteroid hunting mission.
Although STEREO wasn’t designed for this work, the mission already has a team of volunteer near-Earth asteroid hunters at the ready and their optics are more than capable of looking out for large lumps of rock invisible from Earth.
“The close-up investigation of L4 and L5 is completely new. That makes it something we should be driving,” says Richard Harrison of the Rutherford Appleton Laboratory in Oxfordshire, UK and a member of the STEREO project. “Wouldn’t it be spectacular if we actually backed past an asteroid? Saw it come creeping into view around the camera.” Now that would be a huge discovery.
This isn’t simply out of academic curiosity however. The Earth’s Moon is thought to have been formed after a huge cosmic impact with a small planetary body. The problem comes when trying to explain where the offending planetary body could have come from; too far away and it will have had too much energy. Rather than punching into the side of the Earth it would have shattered our planet. So the body must have formed a lot closer to our planet.
Did this body evolve in either the L4 and L5 points? If it did, and then somehow got kicked out of the gravitational island, perhaps careering toward the Earth, causing the cataclysmic impact that seeded the formation of the Moon.
It is exciting to think that STEREO may make some ground-breaking discoveries not Sun related. I just hope they don’t bump in to any chunks of rock, it could be pretty crowded out there…
The X9-class solar flare of Dec. 5, 2006, observed by the Solar X-Ray Imager aboard NOAAs GOES-13 satellite (NASA)
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In 2006, one of the largest solar flares observed for 30 years erupted, saturating X-ray cameras on board observatories orbiting Earth. The December 5th event was a powerful X-ray flare, registering “X9” on the scale of powerful “X-class” flares. Even though flares weighing in at X20+ have been observed, the X9 is a rare event all the same. However, this 2006 flare is fast becoming known not only for its energetic characteristics. Shortly after the flare, solar astronomers expected to see a flood of interplanetary ions being ejected by the Sun. However, they detected something else; not only a particle they weren’t expecting, but a particle that shouldn’t be there…
When a blast the size of a hundred million nuclear bombs detonates, you wouldn’t expect anything to be intact at ground-zero, would you? In the case of solar flares, a huge amount of magnetic energy is unleashed through a process known as reconnection, quickly accelerating and heating solar plasma. Depending on the conditions, different solar flare energies are possible, but in the case of the Dec. 5th 2006 flare, solar plasma was rapidly and violently accelerated, unleashing X-ray radiation. At the flare site, within the knotted and twisted magnetic flux, plasma temperatures can soar to 10-20 million Kelvin (occasionally, for the biggest flares, 100 million Kelvin). In these conditions, nothing stays intact. Any atoms in the local area become stripped of their electrons, leaving an energetic soup of ionized particles (like protons and helium nuclei) and electrons.
So you can imagine the surprise of a group of solar physicists using data from the twin Solar Terrestrial Relations Observatory (STEREO) spacecraft orbiting the Sun (one ahead of the Earth’s orbit, and one behind), when they detected a jet of pure neutral hydrogen atoms emanating from the flare.
“We’ve detected a stream of perfectly intact hydrogen atoms shooting out of an X-class solar flare,” says Richard Mewaldt of Caltech,. “What a surprise! These atoms could be telling us something new about what happens inside flares.”
“No other elements were present, not even helium (the sun’s second most abundant atomic species). Pure hydrogen streamed past the spacecraft for a full 90 minutes.”
STEREO particle counts after the flare (NASA)Measurements of radio emissions indicated that a shock wave had been generated low in the solar atmosphere during the flare, revealing the interaction of incoming solar ions. Physicists waited for an hour for the incoming ions (the time calculated for ions to travel from the Sun to the STEREO spacecraft), but instead the stream of neutral atoms arrived. The stream of hydrogen lasted for 90 minutes, and then it went quiet for 30 minutes only for the expected ions to flood the sensors as predicted.
At first glance, the impossible had been achieved; a solar flare had somehow manufactured, then sorted the neutral hydrogen from the soup of plasma and shot it into space. But this produced a very perplexing puzzle: neutral hydrogen, lots of it, has been detected as a result of a solar flare, and yet these atoms cannot exist in the extreme environment surrounding the flare site. What gives?
Actually, these hydrogen atoms were not generated inside the flare, they formed after the flare as the products from the explosion spiralled into interplanetary space.
“We believe they began their journey to Earth in pieces, as protons and electrons,” said Mewaldt. “Before they escaped the sun’s atmosphere, however, some of the protons recaptured an electron, forming intact hydrogen atoms. The atoms left the sun in a fast, straight shot before they could be broken apart again.”
The reason why these neutral atoms appeared at STEREO faster than the ion cloud is because the neutral hydrogen did not get influenced (slowed down) by the Sun’s magnetic field; the atoms shot out, in a straight line, rather than being deflected by magnetic flux. And how did they form? Physicists believe the protons “recaptured” the free electrons in the space between the flare and detector through the well known mechanisms radiative recombination and charge exchange.
Now, solar physicists want to replicate these findings to see whether these hydrogen jets are a common feature of solar flares… but they might have to wait a while, the Sun is still enjoying its quiet spell...
Modes of solar oscillation plotted over our Sun. Could the same things be done with other stars? (NASA/TRACE/NCAR)
[/caption]Observing a stars brightness pulsate may reveal its internal structure say researchers using the Convection Rotation and Planetary Transits (CoRoT) observatory. The highly sensitive orbital telescope can detect tiny variations in a distant star’s brightness, leading astronomers into a new field of stellar seismology called “asteroseismology.”
Seismology is more commonly used by scientists on Earth to see how waves travel through the terrestrial crust, thereby revealing the structure of the material below us. Even solar physicists use the method of helioseismology to understand the interior of our Sun by observing its wobble. Now, by observing the slight changes in stellar brightness, it is possible to remotely probe deep into the inner workings of a distant star…
CoRoT is a joint French Space Agency (CNES) and European Space Agency (ESA) mission to detect slight variations in the brightness of stars launched in 2006. As extrasolar planets pass in front of (or “transit”) a star, the brightness will decrease. The highly sensitive 27 cm-diameter telescope and spectroscopic instrumentation has the ability of detecting extrasolar rocky planets a few times the size of Earth and new gas giants (a.k.a. Hot Jupiters).
Another mission objective for the 630 kg satellite is to detect luminosity variations associated with acoustic pulsations passing through the body of the star. A similar method known as helioseismology uses the Solar and Heliospheric Observatory (SOHO) to detect the propagation of pressure waves through the Sun so a better idea of solar internal dynamics and structure can be gained.
CoRoT has been watching three stars, 20-40% more massive than the Sun, vibrate in reaction to the convective processes on the stellar surfaces. Some areas will expand and cool, whilst others with contract and heat up. This creates an oscillation, and a pulsation in brightness, providing information about the inner structure of these distant stars. The three stars brightened and dimmed 1.5 times more dramatically than solar helioseismology observations. However, this is still 25% weaker than expected from theory, so it would seem stellar physics still has a long way to go.
“This really marks the start of a completely new era of space-based asteroseismology,” said Joergen Christensen-Dalsgaard of the University of Aarhus in Denmark. “It shows that CoRoT can do what it set out to do.”
Asteroseismology can also be used to gauge the precise age of a star. Usually, the age of a star is determined by looking at a star cluster where it is assumed the majority of the stars are of a similar age. However, as a star ages, different elements undergo nuclear fusion at different times. This alters the star’s interior structure and therefore alters the vibrational characteristics of the star. This can be detected by CoRoT, hopefully aiding astronomers when deducing the precise ago of a particular star.
“In principle, you can look at one star all on its own and determine how old it is,” adds Michael Montgomery of the University of Texas.
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We’ve talked about the Sun before, but this time we’re going to look at the entire life cycle of the Sun, and all the stages it’s going to go through: solar nebula, protostar, main sequence, red giant, white dwarf, and more. Want to know what the future holds for the Sun, get ready for the grim details.
The three Ulysses spacecraft orbits of the Sun. Figure shows radial solar wind velocity and images of the Sun at varying degrees of activity (McComas et al. GRL, 2008)
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Solar wind output is at its lowest since accurate records began 50 years ago. This finding comes from the seasoned ESA/NASA solar probe Ulysses, which completed nearly three polar orbits of the Sun from 1993 to 2008 (it is still functioning today, but at a reduced capacity). Although a weakening of the solar wind may not sound very important, the effects of this reduction will have serious implications, diminishing the natural defences of the heliopause (our Solar System’s invisible barrier) which protects us from high energy cosmic rays blasting through intergalactic space…
The heliosphere (NASA/Feimer)
Ulysses has orbited the Sun four times longer than was originally planned. This tough solar satellite was launched in 1990 on board Space Shuttle Discovery, and in 1992, the probe used Jupiter to slingshot it out of the Solar System’s ecliptic to begin taking in situ measurements of solar wind speed and density at all latitudes from pole-to-pole. This is an unprecedented mission that continues to function today. However, Ulysses’ plutonium fuel in its radioisotope thermoelectric generator (RTG) is dwindling to the point where this landmark mission will die from old age over the coming months.
And yet, the geriatric spaceship still reveals characteristics about our Sun that we could never hope to observe confined to the ecliptic plane. So, in (possibly) one of Ulysses’ biggest discoveries to date, scientists have uncovered the strange phenomenon that the solar wind output has decreased to an all-time low (since accurate records began half a century ago), as the Ulysses Principal Investigator explains:
“The Sun’s 1.5 million km-per-hour solar wind inflates a protective bubble around the Solar System and can influence how things work here on Earth and even out at the boundary of our Solar System, where it meets the galaxy. Ulysses data indicate the solar wind’s global pressure is the lowest we have seen since the beginning of the space age.” – Dave McComas, Principal Investigator for the Ulysses solar wind instrument and senior Executive Director at the Southwest Research Institute in San Antonio, Texas.
This “protective bubble” is also known as the heliosphere, a huge volume of space in which all the planets, asteroids and comets are deep inside. It is the total extent of the Sun’s influence, pushing out into interstellar space, the limit of which is known as the heliopause. The heliopause is formed through a balance between the outward pressure of the solar wind and the inward pressure of the interstellar medium, should one of these pressures fluctuate, the heliopause will expand or contract. Should the solar wind pressure decrease, the heliopause will shrink under the greater interstellar medium pressures. This is exactly what Ulysses has detected: a reduction in solar wind pressure.
So what does this mean to us? The heliopause blocks and deflects the majority of damaging high energy interstellar particles (a.k.a. cosmic rays). Should the solar wind weaken, the heliopause will become a less-effective shield, letting more cosmic rays into the Solar System.
“Galactic cosmic rays carry with them radiation from other parts of our galaxy. With the solar wind at an all-time low, there is an excellent chance that the heliosphere will diminish in size and strength. If that occurs, more galactic cosmic rays will make it into the inner part of our Solar System.” – Ed Smith, NASA’s Ulysses Project Scientist from the Jet Propulsion Laboratory, California.
Artist impression of Ulysses (ESA)
The effects of this happening will be far-reaching and could severely impact the future of manned exploration of the Solar System.
Solar physicists made this discovery when analysing Ulysses data from the probe’s third scan of the solar wind and interplanetary magnetic field (IMF) from the Sun’s north to south poles. On comparison with previous scans, it was found that the solar wind pressure and the radial component of the magnetic field embedded in the solar wind had decreased by 20%. The magnetic field strength surrounding Ulysses had dropped by a huge 36%.
So what could this be attributed to? Physicists simply do not know. Perhaps it might be related to the extended solar minimum in recent months, as Smith appears to suggest. “The sun cycles between periods of great activity and lesser activity,” Smith said. “Right now, we are in a period of minimal activity that has stretched on longer than anyone anticipated.”
Compelling results from a compelling solar mission…
Now you see them... The sunspot group as observed by SOHO MDI today (NASA/SOHO)
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After an extended period of calm for Solar Cycle 24, a cluster of sunspots have appeared on the disk of the Sun. Although we have observed sunspots since the beginning of this new solar cycle (which officially began on January 4th, 2008 with the observation of a high-latitude sunspot pair), this is the first time for many months “new” Cycle 24 sunspots have shown themselves. Before today, the sunspots (including occasional flares and coronal mass ejections) belonged to the previous cycle (Cycle 23). It would appear the spots have evolved into a cluster in a high-latitude location with the magnetic polarity consistent with this new cycle. But does this mean we can expect an increase in solar activity after this pretty dull period of “blank” solar disk observations? Your guess is as good as mine…
Overlapping solar cycles are natural occurrences, and extended solar minima are not unexpected, but many predictions of an extended period of solar calm have been put forward since Solar Cycle 24 appeared to shy away after the initial excitement in January. Although the Sun has been surprisingly quiet for several months, we’ve still had sporadic sunspot activity (plus the occasional flare and CME eruption), but none could be attributed to the new Cycle 24 (although I erroneously thought the August sunspot activity was due to Cycle 24, it was in fact due to the overlapping Cycle 23).
So how can we be so sure these new observations are of Cycle 24 spots and not Cycle 23 spots? After quickly glancing at the Solar and Heliospheric Observatory (SOHO) image (top), we can see a cluster of activity at a fairly high latitude. Generally speaking, one would expect sunspots at the beginning of a new cycle to appear at high latitudes. As the 11-year solar cycle progresses, sunspot activity will begin to drift equator-wards, to lower latitudes. “Old” Cycle 23 sunspots have generally appeared near the solar equator, so the sunspots observed today can be attributed to the “new” Cycle 24.
The clincher for identifying these spots as belonging to a new solar cycle is their magnetic polarity. Sunspots often appear in pairs of opposite polarity (i.e. one will be magnetic north, the other will be magnetic south), and this new cluster is consistent with the polarity expected for Cycle 24 sunspots. SOHO uses its Michelson Doppler Imager (MDI) Magnetogram instrument to observe magnetic polarity, and it would appear that the polarity of this sunspot cluster has an opposite magnetic north/south to previous Cycle 23 observations.
So does this mean we might see an increase in solar activity from here on in? Although this is an encouraging observation, the Sun could revert back to its “blank” state as quickly as it revealed these sunspots to SOHO. However, there is also a chance this could herald the beginning of accelerated solar activity, possibly still fulfilling NASA’s 2006 prediction that Solar Cycle 24 will be a “doozy.”
A Black Brant sounding rocket of the type that will carry SUMI above Earth's atmosphere (NASA)
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Solar physicists will have the unprecedented opportunity to peer inside one of the most mysterious regions in the Sun’s atmosphere. Separating the chromosphere (at a temperature of a few thousand Kelvin) and the extended corona (at a temperature of over a million Kelvin) is a very thin layer about 5000 km above the photosphere (a.k.a. the Sun’s “surface”). The transition region dictates the characteristics of the hot plasma passing from the Sun into space and is right at the start of the solar-terrestrial chain, controlling space weather. We are unable to directly observe the transition region as it doesn’t radiate in wavelengths observable from the Earth’s surface, but it does emit UV radiation observable from space. So a group of solar researchers are packing some very sensitive instrumentation into a sounding rocket that will very briefly take some snapshots of the transition region. But they will have to be quick, from instrument deployment to re-entry, only eight minutes will be allowed to take the necessary UV spectroscopic observations…
To start us off, Jonathan Cirtain from the Marshall Space Flight Center explains this new mission in a nutshell:
“Early next year, we’re going to launch an experimental telescope that can measure vector magnetic fields in the transition region.” – Cirtain.
But why? Hasn’t the transition region been observed before? Actually, no. The surface of the Sun has been tirelessly studied, as has the solar atmosphere, but the thin layer separating the two has, so far, remained hidden from solar astronomers. “Just bad luck, really,” says Cirtain as he explains why the transition region has remained a mystery for so long. “Gas in the transition region doesn’t produce many strong spectral lines that we can see at visible wavelengths.” But it does radiate UV emission that can be observed from space, so Cirtain hopes his research group will be the first to peer right inside by pushing into space.
Coronal loops as viewed by the Transition Region and Coronal Explorer (TRACE)
The transition region is critical to the understanding of the magnetic structure of the Sun and its corona. Below this thin layer, plasma pressure dominates, above it, magnetic pressure dominates. This means that above the transition region, the characteristics of the Sun’s magnetic field overwhelm the decreasing plasma pressure. The corona becomes a highly structured entity from the transition region and upward, which can be seen in the structure of magnetic coronal loops (pictured).
But what will they be measuring? How can the magnetic structure inside the transition region be seen? The instrument to be launched is called the Solar Ultraviolet Magnetograph Investigation (SUMI) and it is designed to measure the magnetic phenomenon of “Zeeman Splitting.” The Zeeman effect occurs when radiating plasma is in the presence of a strong magnetic field. So in the case of SUMI, the instrument will observe the UV emission from the transition region and detect the fine-scale splitting of the UV spectroscopic emission lines. The stronger the magnetic field, the greater the splitting.
SUMI can also measure the polarization of the split lines, so Cirtain’s team will have all the information they’ll ever need about the magnetic field in the transition region: both magnetic filed strength and direction. So far, so good.
But how is Cirtain planning on getting SUMI into space? To possibly make it cheap, and because SUMI is a comparatively simple instrument, it won’t need to be in space long. So the plan is to blast it into the lowest reaches of space on a sub-orbital flight inside the nose cone of a Black Brant sounding rocket. 68 seconds and 300 km (185 miles) into the flight, SUMI will be jettisoned. “We’ll be above more than 99.99% of Earth’s atmosphere. From that moment, we’ve only got 8 minutes to work with. We’ll target an active region and start taking data,” Cirtain added.
Carrying out short sounding rocket missions is not new to solar physics, some of the very first space-based observations of the Sun could only come from high altitude rockets. However, Cirtain will be nervous to see SUMI disappear into the stratosphere at 5,000 mph and has dubbed the flight the “Eight minutes of terror.”
So, from one of the simplest and cheapest observation campaigns, the mysterious transition region may start to give us some answers…
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Amateur astronomers have observed the first sunspots to appear on the solar surface for weeks. This period of extreme magnetic calm has made some scientists believe that Solar Cycle 23 might be a quiet affair. This comes in stark contrast to NASA’s 2006 forecast that this cycle would be a “doozy.” Whether or not the slow start of solar activity is indicative of things to come, we’re not sure, but it sure is great to see activity starting to churn on the solar surface once more…
To make the situation even more cloudy, back in March, we had a false alarm. Suddenly, the Sun erupted to life, only three months after the start of Cycle 24 was announced. Sunspots, flares and Coronal Mass Ejections sprung to life around the solar equator. You would have been forgiven for thinking the Sun was going to make good on the NASA 2006 forecast. But it wasn’t to be. Critically, these active sunspots were “left overs” from the previous cycle. Like revellers turning up an hour after the party had finished, these sunspots were overlapping remainders of the previous cycle.
At the root of all these observations is space weather prediction. All our activities in space are in some way influenced by solar activity, so it would be advantageous if we could predict when the next solar storm is coming. We have complex models of the Sun and our observational skills are becoming more and more sophisticated, but we still have a very basic grasp on what makes the Sun “tick.”
So today’s discovery, although a little overdue, will excite solar physicists and astronomers the world over. But will the solar activity continue? Is this just an isolated occurrence? For now, we just do not know. We have to sit back, observe and enjoy what surprises the Sun has in store for us in Cycle 24.
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We’ve all wondered at some point or another what mysteries our Solar System holds. After all, the eight planets (plus Pluto and all those other dwarf planets) orbit within a very small volume of the heliosphere (the volume of space dominated by the influence of the Sun), what’s going on in the rest of the volume we call our home? As we push more robots into space, improve our observational capabilities and begin to experience space for ourselves, we learn more and more about the nature of where we come from and how the planets have evolved. But even with our advancing knowledge, we would be naive to think we have all the answers, so much still needs to be uncovered. So, from a personal point of view, what would I consider to be the greatest mysteries within our Solar System? Well, I’m going to tell you my top ten favourites of some more perplexing conundrums our Solar System has thrown at us. So, to get the ball rolling, I’ll start in the middle, with the Sun. (None of the following can be explained by dark matter, in case you were wondering… actually it might, but only a little…)
10. Solar Pole Temperature Mismatch
Data from Ulysses (D. McComas)
Why is the Sun’s South Pole cooler than the North Pole? For 17 years, the solar probe Ulysses has given us an unprecedented view of the Sun. After being launched on Space Shuttle Discovery way back in 1990, the intrepid explorer took an unorthodox trip through the Solar System. Using Jupiter for a gravitational slingshot, Ulysses was flung out of the ecliptic plane so it could pass over the Sun in a polar orbit (spacecraft and the planets normally orbit around the Sun’s equator). This is where the probe journeyed for nearly two decades, taking unprecedented in-situ observations of the solar wind and revealing the true nature of what happens at the poles of our star. Alas, Ulysses is dying of old age, and the mission effectively ended on July 1st (although some communication with the craft remains).
However, observing uncharted regions of the Sun can create baffling results. One such mystery result is that the South Pole of the Sun is cooler than the North Pole by 80,000 Kelvin. Scientists are confused by this discrepancy as the effect appears to be independent of the magnetic polarity of the Sun (which flips magnetic north to magnetic south every 11-years). Ulysses was able to gauge the solar temperature by sampling the ions in the solar wind at a distance of 300 million km above the North and South Poles. By measuring the ratio of oxygen ions (O6+/O7+), the plasma conditions at the base of the coronal hole could be measured.
Mars, just a normal planet. No mystery here... (NASA/Hubble)
Why are the Martian hemispheres so radically different? This is one mystery that had frustrated scientists for years. The northern hemisphere of Mars is predominantly featureless lowlands, whereas the southern hemisphere is stuffed with mountain ranges, creating vast highlands. Very early on in the study of Mars, the theory that the planet had been hit by something very large (thus creating the vast lowlands, or a huge impact basin) was thrown out. This was primarily because the lowlands didn’t feature the geography of an impact crater. For a start there is no crater “rim.” Plus the impact zone is not circular. All this pointed to some other explanation. But eagle-eyed researchers at Caltech have recently revisited the impactor theory and calculated that a huge rock between 1,600 to 2,700 km diameter can create the lowlands of the northern hemisphere (see Two Faces of Mars Explained).
Bonus mystery: Does the Mars Curse exist? According to many shows, websites and books there is something (almost paranormal) out in space eating (or tampering with) our robotic Mars explorers. If you look at the statistics, you would be forgiven for being a little shocked: Nearly two-thirds of all Mars missions have failed. Russian Mars-bound rockets have blown up, US satellites have died mid-flight, British landers have pock-marked the Red Planet’s landscape; no Mars mission is immune to the “Mars Triangle.” So is there a “Galactic Ghoul” out there messing with our ‘bots? Although this might be attractive to some of us superstitious folk, the vast majority of spacecraft lost due to The Mars Curse is mainly due to heavy losses during the pioneering missions to Mars. The recent loss rate is comparable to the losses sustained when exploring other planets in the Solar System. Although luck may have a small part to play, this mystery is more of a superstition than anything measurable (see The “Mars Curse”: Why Have So Many Missions Failed?).
8. The Tunguska Event
Artist impression of the Tunguska event (www.russianspy.org)
What caused the Tunguska impact? Forget Fox Mulder tripping through the Russian forests, this isn’t an X-Files episode. In 1908, the Solar System threw something at us… but we don’t know what. This has been an enduring mystery ever since eye witnesses described a bright flash (that could be seen hundreds of miles away) over the Podkamennaya Tunguska River in Russia. On investigation, a huge area had been decimated; some 80 million trees had been felled like match sticks and over 2,000 square kilometres had been flattened. But there was no crater. What had fallen from the sky?
This mystery is still an open case, although researchers are pinning their bets of some form of “airburst” when a comet or meteorite entered the atmosphere, exploding above the ground. A recent cosmic forensic study retraced the steps of a possible asteroid fragment in the hope of finding its origin and perhaps even finding the parent asteroid. They have their suspects, but the intriguing thing is, there is next-to-no meteorite evidence around the impact site. So far, there doesn’t appear to be much explanation for that, but I don’t think Mulder and Scully need be involved (see Tunguska Meteoroid’s Cousins Found?).
7. Uranus’ Tilt
Uranus. Does it on its side (NASA/Hubble)
Why does Uranus rotate on its side? Strange planet is Uranus. Whilst all the other planets in the Solar System more-or-less have their axis of rotation pointing “up” from the ecliptic plane, Uranus is lying on its side, with an axial tilt of 98 degrees. This means that for very long periods (42 years at a time) either its North or South Pole points directly at the Sun. The majority of the planets have a “prograde” rotation; all the planets rotate counter-clockwise when viewed from above the Solar System (i.e. above the North Pole of the Earth). However, Venus does the exact opposite, it has a retrograde rotation, leading to the theory that it was kicked off-axis early in its evolution due to a large impact. So did this happen to Uranus too? Was it hit by a massive body?
Some scientists believe that Uranus was the victim of a cosmic hit-and-run, but others believe there may be a more elegant way of describing the gas giant’s strange configuration. Early in the evolution of the Solar System, astrophysicists have run simulations that show the orbital configuration of Jupiter and Saturn may have crossed a 1:2 orbital resonance. During this period of planetary upset, the combined gravitational influence of Jupiter and Saturn transferred orbital momentum to the smaller gas giant Uranus, knocking it off-axis. More research needs to be carried out to see if it was more likely that an Earth-sized rock impacted Uranus or whether Jupiter and Saturn are to blame.
6. Titan’s Atmosphere
False colour image of Titan's atmosphere. Credit: NASA/JPL/Space Science Institute/ESA
Why does Titan have an atmosphere? Titan, one of Saturn’s moons, is the only moon in the Solar System with a significant atmosphere. It is the second biggest moon in the Solar System (second only to Jupiter’s moon Ganymede) and about 80% more massive than Earth’s Moon. Although small when compared with terrestrial standards, it is more Earth-like than we give it credit for. Mars and Venus are often cited as Earth’s siblings, but their atmospheres are 100 times thinner and 100 times thicker, respectively. Titan’s atmosphere on the other hand is only one and a half times thicker than Earth’s, plus it is mainly composed of nitrogen. Nitrogen dominates Earth’s atmosphere (at 80% composition) and it dominates Titans atmosphere (at 95% composition). But where did all this nitrogen come from? Like on Earth, it’s a mystery.
Titan is such an interesting moon and is fast becoming the prime target to search for life. Not only does it have a thick atmosphere, its surface is crammed full with hydrocarbons thought to be teeming with “tholins,” or prebiotic chemicals. Add to this the electrical activity in the Titan atmosphere and we have an incredible moon with a massive potential for life to evolve. But as to where its atmosphere came from… we just do not know.
5. Solar Coronal Heating
Coronal loops as imaged by TRACE at 171 Angstroms (1 million deg C) (NASA/TRACE)
Why is the solar atmosphere hotter than the solar surface? Now this is a question that has foxed solar physicists for over half a century. Early spectroscopic observations of the solar corona revealed something perplexing: The Sun’s atmosphere is hotter than the photosphere. In fact, it is so hot that it is comparable to the temperatures found in the core of the Sun. But how can this happen? If you switch on a light bulb, the air surrounding the glass bulb wont be hotter than the glass itself; as you get closer to a heat source, it gets warmer, not cooler. But this is exactly what the Sun is doing, the solar photosphere has a temperature of around 6000 Kelvin whereas the plasma only a few thousand kilometres above the photosphere is over 1 million Kelvin. As you can tell, all kinds of physics laws appear to be violated.
However, solar physicists are gradually closing in on what may be causing this mysterious coronal heating. As observational techniques improve and theoretical models become more sophisticated, the solar atmosphere can be studied more in-depth than ever before. It is now believed that the coronal heating mechanism may be a combination of magnetic effects in the solar atmosphere. There are two prime candidates for corona heating: nanoflares and wave heating. I for one have always been a huge advocate of wave heating theories (a large part of my research was devoted to simulating magnetohydrodynamic wave interactions along coronal loops), but there is strong evidence that nanoflares influence coronal heating too, possibly working in tandem with wave heating.
Although we are pretty certain that wave heating and/or nanoflares may be responsible, until we can insert a probe deep into the solar corona (which is currently being planned with the Solar Probe mission), taking in-situ measurements of the coronal environment, we won’t know for sure what heats the corona (see Warm Coronal Loops May Hold the Key to Hot Solar Atmosphere).
4. Comet Dust
Comets - where does their dust come from?
How did dust formed at intense temperatures appear in frozen comets? Comets are the icy, dusty nomads of the Solar System. Thought to have evolved in the outermost reaches of space, in the Kuiper Belt (around the orbit of Pluto) or in a mysterious region called the Oort Cloud, these bodies occasionally get knocked and fall under the weak gravitational pull of the Sun. As they fall toward the inner Solar System, the Sun’s heat will cause the ice to vaporize, creating a cometary tail known as the coma. Many comets fall straight into the Sun, but others are more lucky, completing a short-period (if they originated in the Kuiper Belt) or long-period (if they originated in the Oort Cloud) orbit of the Sun.
But something odd has been found in the dust collected by NASA’s 2004 Stardust mission to Comet Wild-2. Dust grains from this frozen body appeared to have been formed a high temperatures. Comet Wild-2 is believed to have originated from and evolved in the Kuiper Belt, so how could these tiny samples be formed in an environment with a temperature of over 1000 Kelvin?
The Solar System evolved from a nebula some 4.6 billion years ago and formed a large accretion disk as it cooled. The samples collected from Wild-2 could only have been formed in the central region of the accretion disk, near the young Sun, and something transported them into the far reaches of the Solar System, eventually ending up in the Kuiper Belt. But what mechanism could do this? We are not too sure (see Comet Dust is Very Similar to Asteroids).
3. The Kuiper Cliff
The bodies in the Kuiper Belt (Don Dixon)
Why does the Kuiper Belt suddenly end? The Kuiper Belt is a huge region of the Solar System forming a ring around the Sun just beyond the orbit of Neptune. It is much like the asteroid belt between Mars and Jupiter, the Kuiper Belt contains millions of small rocky and metallic bodies, but it’s 200-times more massive. It also contains a large quantity of water, methane and ammonia ices, the constituents of cometary nuclei originating from there (see #4 above). The Kuiper Belt is also known for its dwarf planet occupant, Pluto and (more recently) fellow Plutoid “Makemake”.
The Kuiper Belt is already a pretty unexplored region of the Solar System as it is (we wait impatiently for NASA’s New Horizons Pluto mission to arrive there in 2015), but it has already thrown up something of a puzzle. The population of Kuiper Belt Objects (KBOs) suddenly drops off at a distance of 50 AU from the Sun. This is rather odd as theoretical models predict an increase in number of KBOs beyond this point. The drop-off is so dramatic that this feature has been dubbed the “Kuiper Cliff.”
We currently have no explanation for the Kuiper Cliff, but there are some theories. One idea is that there are indeed a lot of KBOs beyond 50 AU, it’s just that they haven’t accreted to form larger objects for some reason (and therefore cannot be observed). Another more controversial idea is that KBOs beyond the Kuiper Cliff have been swept away by a planetary body, possibly the size of Earth or Mars. Many astronomers argue against this citing a lack of observational evidence of something that big orbiting outside the Kuiper Belt. This planetary theory however has been very useful for the doomsayers out there, providing flimsy “evidence” for the existence of Nibiru, or “Planet X.” If there is a planet out there, it certainly is not “incoming mail” and it certainly is notarriving on our doorstep in 2012.
So, in short, we have no clue why the Kuiper Cliff exists…
2. The Pioneer Anomaly
Artist impression of the Pioneer 10 probe (NASA)
Why are the Pioneer probes drifting off-course? Now this is a perplexing issue for astrophysicists, and probably the most difficult question to answer in Solar System observations. Pioneer 10 and 11 were launched back in 1972 and 1973 to explore the outer reaches of the Solar System. Along their way, NASA scientists noticed that both probes were experiencing something rather strange; they were experiencing an unexpected Sun-ward acceleration, pushing them off-course. Although this deviation wasn’t huge by astronomical standards (386,000 km off course after 10 billion km of travel), it was a deviation all the same and astrophysicists are at a loss to explain what is going on.
One main theory suspects that non-uniform infrared radiation around the probes’ bodywork (from the radioactive isotope of plutonium in its Radioisotope Thermoelectric Generators) may be emitting photons preferentially on one side, giving a small push toward the Sun. Other theories are a little more exotic. Perhaps Einstein’s general relativity needs to be modified for long treks into deep space? Or perhaps dark matter has a part to play, having a slowing effect on the Pioneer spacecraft?
How do we know the Oort Cloud even exists? As far as Solar System mysteries go, the Pioneer anomaly is a tough act to follow, but the Oort cloud (in my view) is the biggest mystery of all. Why? We have never seen it, it is a hypothetical region of space.
At least with the Kuiper Belt, we can observe the large KBOs and we know where it is, but the Oort Cloud is too far away (if it really is out there). Firstly, the Oort Cloud is predicted to be over 50,000 AU from the Sun (that’s nearly a light year away), making it about 25% of the way toward our nearest stellar neighbour, Proxima Centauri. The Oort Cloud is therefore a very long way away. The outer reaches of the Oort Cloud is pretty much the edge of the Solar System, and at this distance, the billions of Oort Cloud objects are very loosely gravitationally bound to the Sun. They can therefore be dramatically influenced by the passage of other nearby stars. It is thought that Oort Cloud disruption can lead to icy bodies falling inward periodically, creating long-period comets (such as Halley’s comet).
In fact, this is the only reason why astronomers believe the Oort Cloud exists, it is the source of long-period icy comets which have highly eccentric orbits emanating regions out of the ecliptic plane. This also suggests that the cloud surrounds the Solar System and is not confined to a belt around the ecliptic.
So, the Oort Cloud appears to be out there, but we cannot directly observe it. In my books, that is the biggest mystery in the outermost region of our Solar System…
NASA’s twin STEREO spacecraft have been studying the sun since their launch in 2006. But the mission made a surprising and unexpected discovery by detecting particles from the edge of the solar system, and for the first time, scientists have now been able to map the region where the hot solar wind meets up with the cold interstellar medium. However, this wasn’t done with optical instruments imaging in visible light, but by mapping the region by means of neutral, or uncharged, atoms. This breakthrough is a “new kind of astronomy using neutral atoms,” said Robert Lin, from the University of California Berkeley, and lead for the suprathermal electron sensor aboard STEREO. “You can’t get a global picture of this region, one of the last unexplored regions of the heliosphere, any other way because it is too tenuous to be seen by normal optical telescopes.” The findings also help clear up a discrepancy in the amount of energy in the region found by the Voyager 2 spacecraft as it passed through the edge of the solar system last year.
The heliosphere stretches from the sun to more than twice the distance of Pluto. Beyond its edge, called the heliopause, lies the relative quiet of interstellar space, at about 100 astronomical units (AU) – 100 times the Earth-sun distance. The termination shock is the region of the heliosphere where the supersonic solar wind slows to subsonic speed as it merges with the interstellar medium. The heliosheath is the region of churning plasma between the shock front and the interstellar medium.
The twin STEREO spacecraft, in Earth’s orbit about the sun, take stereo pictures of the sun’s surface and measure magnetic fields and ion fluxes associated with solar explosions.
Between June and October 2007, however, the suprathermal electron sensor in the IMPACT (In-situ Measurements of Particles and CME Transients) suite of instruments on board each STEREO spacecraft detected neutral atoms originating from both the shock front and the heliosheath beyond.
“The suprathermal electron sensors were designed to detect charged electrons, which fluctuate in intensity depending on the magnetic field,” said lead author Linghua Wang, a graduate student in UC Berkeley’s Department of Physics. “We were surprised that these particle intensities didn’t depend on the magnetic field, which meant they must be neutral atoms.”
UC Berkeley physicists concluded that these energetic neutral atoms were originally ions heated up in the termination that lost their charge to cold atoms in the interstellar medium and, no longer hindered by magnetic fields, flowed back toward the sun and into the suprathermal electron sensors on STEREO.
“This is the first mapping of energetic neutral particles from beyond the heliosphere,” Lin said. “These neutral atoms tell us about the hot ions in the heliosheath. The ions heated in the termination shock exchange charge with the cold, neutral atoms in the interstellar medium to become neutral, and then flow back in.”
According to Lin, the neutral atoms are probably hydrogen, since most of the particles in the local interstellar medium are hydrogen.
The findings from STEREO, reported in the July 3 issue of the journal Nature, clear up a discrepancy in the amount of energy dumped into space by the decelerating solar wind that was discovered last year when Voyager 2 crossed the solar system’s termination shock and entered the surrounding heliosheath.
The newly discovered population of ions in the heliosheath contains about 70 percent of the energy dissipated in the termination shock, exactly the amount unaccounted for by Voyager 2’s instruments, the UC Berkeley physicists concluded. The Voyager 2 results are reported in the same issue of Nature.
A new NASA mission, the Interstellar Boundary Explorer (IBEX), is planned for launch later this year to map more thoroughly the lower-energy energetic ions in the heliosheath by means of energetic neutral atoms to discover the structure of the termination shock and how hydrogen ions are accelerated there.
